This invention relates to the production of chirped Bragg grating optical fibre filters. A particular, though not necessarily exclusive, application for such filters is to be found in the provision of optical fibre dispersion equaliser, as for instance described in U.S. Past. No. 4,953,939, to which attention is directed. U.S. Pat. No. 4,953,939 describes a way of making a uniform pitch distributed Bragg grating in an optical fibre by launching into it a high power beam of light and by using a reflector to produce a standing wave pattern in the fibre. An alternative way of making such a uniform pitch grating is to illuminate the fibre from the side with an interference pattern of alternate high and low intensity bands of light. In both instances the resulting grating will normally be of uniform pitch because standing waves and interference patterns are normally of uniform pitch. On the other hand, for many applications, such as in dispersion equalisation, a chirped pitch is required. U.S. Pat. No. 4,953,939 describes a number of different ways in which such chirping can be attained. The present invention is directed to a novel way of providing chirping. U.S. Pat. No. 4,807,950 also relates to the making of uniform pitch distributed Bragg gratings in optical fibres. It does not make any reference to chirping such gratings, but it does refer to attaching to a flexible plate a fibre having a set of different Bragg gratings at different positions along its length. Subsequent bending of the plate subjects each of these gratings to a uniform strain so as to change the pitch of each, and hence its Bragg reflection wavelength. The value of the strain may be different for different members of the set by virtue of bending the plate to a different radius of curvature in each region registering with a different member of the set of gratings.